In recent years, there is a remarkable trend to provide an internal combustion engine for an automobile with a turbocharger to improve fuel consumption. Among several types of turbochargers is a variable geometry turbocharger including variable nozzles disposed on the inlet side of an expansion turbine, whereby a flow rate of a working fluid is adjustable by changing the opening degree of the variable nozzles. Thus, the variable geometry turbocharger can be operated in accordance with a change in the loads of an internal combustion engine, and a response performance is particularly high during low-load operation.
However, as depicted in FIG. 8, a variable geometry turbocharger is characterized in that, when the opening degree of the variable nozzles is small, the turbine efficiency considerably decreases from a peak point (where the nozzle opening degree is near the intermediate opening-degree range). The turbine efficiency during the time when the nozzle opening degree is in the small opening-degree range affects the response performance considerably. Thus, it is desirable to improve the turbine efficiency in the small opening-degree range.
To improve the turbine efficiency in the small opening-degree range, it is advantageous to reduce the incidence angle of the variable nozzles (an incidence angle is a difference between an inflow angle of a working fluid flowing into the variable nozzles and a leading-edge blade angle of the variable nozzles). Thus, to reduce the incidence angle, one may narrow a scroll section (that is, reduce the size of the scroll section) of a turbine housing in accordance with the leading-edge blade angle of the variable nozzles in the small opening-degree range.
On the other hand, narrowing the scroll section leads to an increase in the incidence angle during the time when the nozzle opening degree is in the large opening-degree range, which may raise a risk of flow separation on the blade surfaces of the variable nozzles, a decrease in the actual flow-path area, and reduction in the flow rate of the working fluid (maximum flow rate).
As described above, it is not easy to achieve both an improved turbine efficiency in the small opening-degree range of the variable nozzles and a sufficient maximum flow rate in the large opening-degree range of the variable nozzles, by simply adjusting the shape of the scroll section of the turbine housing. It is thus desirable to make use of techniques other than modification of the shape of the scroll section to satisfy the above two requirements.
Although not intended to achieve both an improved turbine efficiency in the small opening-degree range of the variable nozzles and a sufficient maximum flow rate in the large opening-degree range, Patent Document 1 discloses a turbine including an annular guide portion (guide surface) formed on an inlet side of turbine blades on a wall surface of a shroud of a turbine housing facing tips of the turbine blades, the guide portion being inclined toward the back-surface side of a turbine wheel in the radial direction of the turbine.
With the turbine disclosed in Patent Document 1, a working fluid flowing into the turbine blades is attracted to the back-surface side of the turbine wheel by formation of the guide portion, which suppresses an exciting force generated by wake of the variable nozzles acting in the vicinity of the tips of the turbine blades, and thereby suppresses vibration of the turbine blades. Furthermore, a gap between the turbine blades and the wall surface of the shroud is blocked by the guide portion, and thereby a clearance flow via the gap is suppressed.